It is known that a panel may be made to function as a loudspeaker by attaching to the panel an audiofrequency actuator which can transmit acoustic signals into the panel. WO92/03024 discloses one such panel. WO98/52289 suggests the use of standard trim panels in a motor vehicle as a means of radiating the output of a radio, tape player or the like, listing magnetostrictive, electromagnetic, electrostatic and micromotor transducers as possible for the job, although preferring piezoelectric transducers.
While such panels can be made to operate as satisfactory loudspeakers, they all depend on attachment of the actuator to the panel, and the availability of mains power or a vehicle alternator output to drive them, for various reasons. Piezoelectric devices require high voltages and relatively large currents to produce suitable levels of audio power, while electromagnetic devices, although operating at low voltages, require very high currents to produce sufficient power to produce audible output from the panels. Magnetostrictive devices can generate a substantial force at a relatively low voltage, but have previously been impractical for portable domestic uses for two reasons. Firstly, the current consumption to generate the necessary magnetic fields to cause change in length of the giant magnetostrictive material (GMM) is relatively high, and secondly, the cost of the GMM would be prohibitive.
It has been found that simply scaling down conventional magnetostrictive actuators does not produce satisfactory audio output, with high levels of distortion or very low volume.